sperm

Also known as spermatozoa or spermia. Male gamete or sex cell, posessing a motile flagellum for movement and a pointed head consisting mainly of nucleus. The base of the flagellum is lined with numerous mitochondria for powering movement; these organelles are discarded before the sperm penetrates the ovum. Enzymes present in the acrosome or 'nose' part of the sperm act on the outer parts of the ovum, altering it's properties so that a sperm may evenually penetrate - note that the enzymes from many hundreds of sperm are required for this task.

Sperm cels are the smallest in human body (roughly 175,000 sperm are needed to match the weight of one egg cell) and here are about 60,000 sperm per mm^3 of ejaculate. Sperm may survive outside the body for several days. They can survive in the fallopian tubes for a similar amount of time, and can be (and have been) extracted alive from the ejaculatory ducts some days after death.

Acidic environments, like those found in the vagina. This is why ejaculate is basic, to help neutralize the deadly acid.

Heat. This is why the testes hang down from the body, to keep them cooler.

History:
It was once believed by sermists that ALL the heritable information was contained in the sperm. Shortly after the invention of the microscope (not a terribly powerful microscope), the spermists thought it had been proven once and for all. When they looked through the microscope at the sperms they saw a tiny little fetus in the head of each one.
Then the microscopes got more powerful...